Australia: The Land Where Time Began

A biography of the Australian continent 

Molecular clouds

There is a large quantity of gas and dust throughout the Universe, and nebulae are formed in locations where there are higher concentrations of this material. The locations of these nebulae are not entirely random. Molecular clouds are the areas that give rise to the formation of stars. These are cold clouds, possibly only a few degrees above absolute zero, and they occupy enormous regions of space.

Molecular clouds allow the formation of several molecules, such as carbon monoxide (CO), water (H₂O), and hydrogen molecules (H₂), due to the conditions within the clouds. As a result of the low temperature molecular hydrogen, the most abundant molecule in the cloud is very difficult to observe. When certain portions of the cloud are 10-30 K above absolute zero CO can be detected. In 1974 traces of carbon monoxide were found in the Milky Way by 2 astronomers, Philip Solomons and Nicholas Scoville, and made possible the finding of molecular clouds.

Molecular clouds have since been found to be huge, and to contain vast amounts of hydrogen. These clouds can range from 10⁵ to 2 x 10⁶ solar masses, and have diameters that range from 12 to 120 pc (parsecs), which is about 40 to390 light years. It is believed that in the Milky Way galaxy the total mass of molecular clouds is about 5 billion solar masses. The density of the hydrogen and dust in these clouds is so low that there are about 200-300 hydrogen molecules per cubic centimetre (cm³). Even though it is several thousand times greater than the average density of matter in the Milky Way, it is still a very low density. It is 10¹⁷ less dense than the atmosphere of the Earth.

It has been deduced that molecular clouds and CO emissions are linked intimately, and areas of star formation can be found by looking at parts of the Milky Way where CO emissions are originating. By comparison with the remainder of the interstellar medium are heavy and dense, therefore there is a tendency for them to settle towards the central layers of the Milky Way. This has produced the phenomenon of dark bands that are seen in the Milky Way. It has been found that the molecular clouds where star formation is occurring outline the spiral lines of the Galaxy, and are about 1,000 pc apart, and strung out along the arms like pearls on a necklace. Star formation also occurs in other places than the spiral arms of galaxies. Stars can form by several other mechanisms.

Sources & Further reading

1. Inglis, Michael, 2015, Astrophysics is Easy, An Introduction for the Amateur Astronomer, 2^nd Edition, Springer International Publishing.